The cross groove constant velocity universal joint is of a type, in which ball grooves, defined in an inner ring, and ball grooves defined in an outer ring, which are paired with the ball grooves in the inner ring, are slanted in respective directions reverse to each other relative to an axis and a torque transmission ball is retained at a crossing area where each of the ball grooves in the inner ring and the respective ball groove in the outer ring are crossed with each other. Because of the structure described above, it is possible to minimize rattling motions between the torque transmission balls and the ball grooves and, in particular, the cross groove constant velocity universal joint is employed in association with a drive shaft of an automotive vehicle in which the rattling motions should be suppressed.
E. R. Wagner, in his “Universal Joint and Driveshaft Design Manual”, SAE, 1991, p. 163-166, (hereinafter referred to as the non-patent document) discloses the most fundamental cross groove constant velocity universal joint.
This non-patent document describes that the number of rolling elements is four or more, generally six, and, under the condition, in which the constant velocity universal joint assumes the maximum working angle, the slant angle between the ball grooves is so chosen that the ball grooves defined in the inner ring and the outer ring, respectively, so as to oppose to each other will not assume a parallel relation to each other. The non-patent document also describes that the crossing angle β defined between respective axes of the inner and outer rings is generally within the range of 13 to 19°.
The Japanese Laid-open Patent Publication No. H05-231435 (hereinafter referred to as the patent document) suggests that, in order to avoid reduction of the maximum working angle when the crossing angle between the ball grooves relative to the axis is reduced, the ball grooves are slanted not only relative to the axis, but also in a plane containing the axis.
The cross groove constant velocity universal joint is generally considered having the maximum working angle that cannot be increased. This is because the wedge angle defined by the ball grooves in the inner and outer rings has a reversible angle (limit angle) when the joint assumes a large working angle. It is considered that if the working angle of the joint exceeds the limit angle, a cage holding the balls is no longer able to assume a load balance and will become unstable enough to permit the constant velocity universal joint to lose its functionality. This phenomenon is confirmed with the standard universal joint having six torque transmission balls, and the limit angle is generally understood as determined by the contact angle α and the crossing angle β of the ball grooves.
In the patent document referred to above, it is formulated that the limit angle can be increased when the ball grooves are slanted in the plane containing the axis. However, it will assume a very difficult shape in terms of manufacture and quality control.
In the cross groove constant velocity universal joint, the wedge angle is formed at the crossing area, where each of the ball grooves in the inner ring and the respective ball groove in the outer ring that is paired with such ball groove in the inner ring at the time the torque is transmitted. By the effect of this wedge angle, the torque transmission ball in each crossing area is apt to run out of such crossing area, but is urged against a pocket face in the cage. Since the inner and outer rings are formed with the respective grooves slanted reversely relative to each other with respect to the axis and the reversely slanted grooves are arranged so as to alternate in a circumferential direction, the neighboring balls tend to move out of the respective crossing area in directions opposite to each other. For this reason, the cage is positioned by the balls. Each of the crossing area of the grooves forms bisectional faces of the working angle at all times. Accordingly, the torque transmission balls are retained at the respective crossing areas of the ball grooves at all times and, even when a displacement in angle occurs between the inner and outer rings, they are maintained within the bisectional faces of the ball grooves at all times. Thus, the cross groove constant velocity universal joint has a velocity-constancy characteristic and is an excellent universal joint with minimized rattling motion.
However, the cross groove constant velocity universal joint is incapable of assuming a relatively large working angle as compared with a constant velocity universal joint of a type, in which the torque transmission balls are controlled by offsetting the center of arcuate ball grooves formed respectively in the inner and outer rings so as to extend axially. This is because when the large working angle is to be assumed, the wedge angle referred to above is inverted enough to disrupt the balance of the force acting from the torque transmission balls to the cage. As a result, the cage fails to maintain the balance of the force and eventually loses the stability.
It may, however, be contemplated to increase the crossing angles β between the ball grooves in the inner or outer rings and the axis of the inner or outer ring to thereby avoid the inversion of the wedge angles. However, since the inner and outer rings are such that the ball grooves, which are slanted in respective directions opposite to each other with respect to the axis, are alternately arranged in the circumferential direction, increase of the crossing angles β is limited because of the necessity to avoid interference between the neighboring ball grooves.
The crossing angles β in the cross groove constant velocity universal joint, which are defined between the ball grooves in the inner or outer rings and the axis of the inner or outer ring, are associated with the sliding stroke of the constant velocity universal joint and, therefore, reduction of the crossing angles β is effective to increase the stroke.
However, reduction of the crossing angle β in order to secure the increased sliding stroke of the constant velocity universal joint results in reduction of the maximum working angle of the constant velocity universal joint. The maximum working angle referred to previously is the angle at which, when the joint once bent is operated to return to the initial position while held under a non-rotating condition, a phenomenon in which the extremely high torque acts. In the worst case it may occur, the angle may freeze and will not return to the initial angle, that is, the phenomenon in which the locking or scratching may occur. Such locking during the bending poses a problem particularly when the constant velocity universal joint is to be installed onto the automotive vehicle.
When the constant velocity universal joint is to be installed on the automotive vehicle, the constant velocity universal joint once bent must be returned to the initial position. Because of this, if the working angle is small and the locking occurs when the constant velocity universal joint is bent, the workability in installing the constant velocity universal joint onto the automotive vehicle is lowered considerably.